Makefile: make mandir, htmldir and infodir absolute

Makefile: make mandir, htmldir and infodir absolute

This matches the use of the variables with the same names in autotools,
reducing the potential for user surprise.

Using relative paths in these variables also causes issues if they are
exported from the Makefile, as discussed in commit c09d62f (Makefile: do
not export mandir/htmldir/infodir, 2013-02-12).

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>